1. Field of the Invention
The present invention relates to a surface light source device, and more particularly, to a high quality, high-uniformity surface light source device using a light emitting diode (LED) and a direct-type backlight unit having the surface light source device.
2. Description of the Related Art
Recently, with a thinner and higher-performing trend of a display device, a liquid crystal display has been broadly utilized in TVs or monitors. A liquid crystal panel does not emit light on its own, thereby requiring a separate light source unit, i.e., a backlight unit (BLU). The BLU has adopted as its light source a cold cathode fluorescent lamp (CCFL), which, however, is giving way to a light emitting diode (LED) lately thanks to its advantage in terms of color representation and power consumption.
In general, the BLU includes an edge-type BLU and a direct-type BLU. In the former, a bar-shaped light source is disposed at an edge of a liquid crystal panel to irradiate light onto a light guide plate. In contrast, in the latter, a surface light source is disposed below the liquid crystal panel to directly irradiate light onto the light guide panel. The direct-type BLU includes a surface light source disposed in a lower part thereof and an optical member such as a diffusing plate disposed at a predetermined distance above the surface light source.
A plurality of red, green and blue LEDs are arranged uniformly at a predetermined interval on a circuit board to realize a white surface light source for a direct-type BLU. In this direct-type BLU, light outputted from the LEDs may slightly differ in uniformity depending on arrangement thereof but typically, color stains are caused at side portions or a central portion of an LCD screen, or a top surface of the BLU. To reduce color non-uniformity including color stains, a greater number of LEDs may be more densely arranged, which however entails higher costs without considerably lessening color non-uniformity at the side portions of the screen.
FIG. 1a is a plan view illustrating an example of a conventional surface light source for a direct-type backlight unit, and FIG. 1b is a schematic view illustrating a light output pattern on the top surface of a backlight unit including the surface light source shown in FIG. 1a. Referring to FIG. 1a, the surface light source device 10 includes a plurality of red R, green G and blue B LEDs arranged on a circuit board 11. Light from these red R, green G and blue B LEDs is combined together to produce white light. A reflective plate 17 is disposed at edges of the board 11 to upwardly reflect light which is propagating laterally from the board 11.
The red R, green G and blue B LEDs are arranged into a cluster 15 with a triangular configuration, and a plurality of the LED cluster 15 are arranged in a matrix. Herein, a light emitting diode (LED) cluster means a unit LED group in which at least two LEDs are clustered to generate white light. As shown in FIG. 1a, all the LED clusters 15 are identically arranged in such a way that the highly visible red LED R in each of the clusters 15 is located top from a center of the triangular configuration (refer to an arrow in the left side of FIG. 1a).
However, in the aforesaid LED arrangement, color stains appear at side portions of the board, specifically, upper side portion S1 and lower side portion S3 of the board due to non-uniform color combination and light reflection in the vicinity of the reflective plate 17. This causes output light to be distributed with less uniformity on an entire top surface 50 of the BLU. That is, as shown in FIG. 1b, a red color stain appears on an area P1 of the top surface of the BLU, i.e., the upper side portion S1 of the board. Meanwhile, a blue or green color stain occurs in another area P2 of the top surface of the BLU, i.e., the lower side portion S3 of the board. Especially, the red light is highly visible so that the red color stain is mainly responsible for degrading color uniformity considerably.
FIG. 2a is a plan view illustrating another example of a conventional surface light source 20 for a backlight unit. FIG. 2b is a schematic view illustrating a light output pattern on a top surface of the backlight unit including the surface light source device of FIG. 2a. Referring to FIG. 2a, LED clusters are grouped into two types of clusters 25 and 26. In the clusters 25, red LEDs are located right from a center of a triangular configuration. In the clusters 26, red LEDs are located left from a center of the triangular configuration (refer to an arrow in FIG. 2a). These two types of LED clusters 25 and 26 are arranged alternately and repeatedly from top to bottom. For example, odd rows have only the first type LED clusters 25 arranged repeatedly and even rows have only the second type LED clusters 26 arranged repeatedly.
The surface light source device of FIG. 2a moderately alleviates color stains caused on the upper side portion S1 and the lower side portion S3 of the board. However, such color stains still appear considerably at corners of the board, notably, an upper right corner and a lower left corner. That is, as shown in FIG. 2b, the corners P3 and P4 of the top surface of the BLU experience a red or yellow color stain. Various alternatives to the arrangement of the LEDs as shown in FIGS. 1a and 2a have been suggested, however, without fundamentally overcoming a problem of color non-uniformity such as color stains.